Asynchronous Processor

ABSTRACT

Systems and methods for executing program instructions in a data processor at a variable rate. In one embodiment, a processor is configured to examine received instructions, identify an execution time associated with each instruction, and generate clock pulses at necessary intervals to obtain the appropriate execution time for each instruction. Instructions may be associated with types or “bins” that are in turn associated with corresponding execution times. The clock pulses may be generated by routing successive pulses through circuits that delay the pulses by desired amounts of time. The processor may also be configured to identify instructions which are input/output (I/O) instructions and are initiated or terminated by completion of handshake procedures and therefore have execution times that vary from one instance to the next.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 60/673.994, filed Apr. 22, 2005, U.S. Provisional PatentApplication 60/674,070, filed Apr. 22, 2005, and U.S. Provisional PatentApplication 60/673,995, filed Apr. 22, 2005. All of the foregoing patentapplications are incorporated by reference as if set forth herein intheir entirety.

BACKGROUND

1. Field of the invention

The invention relates generally to electronic logic circuits, and moreparticularly to systems and methods for processing data using aprocessor that executes program instructions at a variable frequency.

2. Related art

As computer technologies have advanced, the amount of processing powerand the speed of computer systems has increased. The speed with whichsoftware programs can be executed by these systems has therefore alsoincreased. Despite these increases, however, there has been a continuingdesire to make software programs execute faster.

The need for speed is sometimes addressed by hardware acceleration.Conventional processors re-use the same hardware for each instruction ofa sequential program. Frequently, programs contain critical code inwhich the same or similar sections of software are executed many timesrelative to most other sections in an application. To accelerate aprogram, additional hardware is added to provide hardware parallelismfor the critical code fragments of the program. This gives the effect ofsimultaneous execution of all of the instructions in the critical codefragment, depending on the availability of data. In addition, it may bepossible to unroll iterative loops so that separate iterations areperformed at the same time, further accelerating the software.

While there is a speed advantage to be gained, it is not free. Hardwaremust be designed specifically for the software application in question.The implementation of a function in hardware generally takes a greatdeal more effort and resources than implementing it in software.Initially, the hardware architecture to implement the algorithm must bechosen based on criteria such as the operations performed and theircomplexity, the input and output data format and throughput, storagerequirements, power requirements, cost or area restrictions, and otherassorted criteria.

A simulation environment is then set up to provide verification of theimplementation based on simulations of the hardware and comparisons withthe software. A hardware target library is chosen based on the overallsystem requirements. The ultimate target may be an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), orother similar hardware platform. The hardware design then commencesusing a hardware description language (HDL), the target library, and thesimulation environment. Logic synthesis is performed on the HDL designto generate a netlist that represents the hardware based on the targetlibrary.

While there are number of complex and expensive design tools employedthroughout the process, frequent iterations are typically needed inorder to manage tradeoffs, such as between timing, area, power andfunctionality. The difficulty of the hardware design process is afunction of the design objectives and the target library. The continuedadvances in semiconductor technology continue to raise the significanceof device parameters with each new process generation. That, coupledwith the greater design densities that are made possible, ensures thatthe hardware design process will continue to grow in complexity overtime.

This invention pertains to the implementation of algorithms inhardware—hardware that performs logic or arithmetic operations on data.Currently available methodologies range from using single processors,arrays of processors, either fixed (gate array) or field-programmablegate arrays (FPGA), or standard cell (ASIC) or full custom designtechniques. Some designs may combine elements of more than onemethodology. For example, a processor may incorporate a block of fieldprogrammable logic.

When comparing different implementations of programmable logic, thenotion of granularity is sometimes used. It relates to the smallestprogrammable design unit for a given methodology. The granularity mayrange from transistors, through gates and more complex blocks, to entireprocessors. Another consideration in comparing programmable hardwarearchitectures is the interconnect arrangement of the programmableelements. They may range from simple bit-oriented point-to-pointarrangements, to more complex shared buses of various topologies,crossbars, and even more exotic schemes.

Full custom or standard cell designs with gate-level granularity anddense interconnects offer excellent performance, area, and powertradeoff capability. Libraries used are generally gate and registerlevel. Design times can be significant due to the design flow imposed bythe diversity of complex tools required. Verification after layout forfunctionality and timing are frequently large components of the designschedule. In addition to expensive design tools, manufacturing toolingcosts are very high and climbing with each new process generation,making this approach only economical for either very high margin or veryhigh volume designs. Algorithms implemented using full custom orstandard cell techniques are fixed (to the extent anticipated during theinitial design) and may not be altered.

The design methodology for fixed or conventional gate arrays is similarto that of standard cells. The primary advantages of conventional gatearrays are time-to-market and lower unit cost, since individual designsare based on a common platform or base wafer. Flexibility and circuitdensity may be reduced compared to that of a custom or standard celldesign since only uncommitted gates and routing channels are utilized.Like those built with custom or standard cell techniques, algorithmsimplemented using conventional gate arrays are fixed and may not bealtered after fabrication.

FPGAs, like conventional gate arrays, are based on a standard design,but are programmable. In this case, the standard design is a completedchip or device rather than subsystem modules and blocks of uncommittedgates. The programmability increases the area of the deviceconsiderably, resulting in an expensive solution for some applications.In addition, the programmable interconnect can limit the throughput andperformance due to the added impedance and associated propagationdelays. FPGAs have complex macro blocks as design elements rather thansimple gates and registers. Due to inefficiencies in the programmablelogic blocks, the interconnect network, and associated buffers, powerconsumption can be a problem. Algorithms implemented using FPGAs may bealtered and are therefore considered programmable. Due to theinterconnect fabric, they may only be configured when inactive (withoutthe clock running). The time needed to reprogram all of the necessaryinterconnects and logic blocks can be significant relative to the speedof the device, making real-time dynamic programming unfeasible.

Along the continuum of hardware solutions for implementing algorithmslie various degrees of difficulty or specialization. This continuum islike an inverted pyramid, in that the lowest levels require the highestdegree of specialization and hence represent a very small base ofpotential designers, while the higher levels utilize more generallyknown skills and the pool of potential designers grows significantly(see Table 1.) Also, it should be noted that lower levels of thisordering represent lower levels of design abstraction, with levels ofcomplexity rising in higher levels. TABLE 1 Designer bases of differenttechnologies

There is therefore a need for a technology to provide softwareacceleration that offers the speed and flexibility of an ASIC, with theease of use and accessibility of a processor, thus enabling a largedesign and application base.

SUMMARY OF THE INVENTION

This disclosure is directed to systems and methods for executing programinstructions in a data processor at a variable rate. In one embodiment,a processor is configured to examine received instructions and toidentify an execution time associated with each instruction. Forexample, instructions may be associated with one of three types that arerequire short, medium, or long times to execute. The types may be named“fast,” “medium” and “slow,” respectively. The processor generates clockpulses that are routed through fast, medium or slow delay circuits toachieve the appropriate execution time for the correspondinginstruction. The processor is also configured to identify instructionsthat are input/output (I/O) instructions and have execution times thatare dependent upon completion of handshake procedures instead ofpredetermined delays.

An alternative embodiment comprises a method implemented in a dataprocessor. The method includes receiving a program instruction andexecuting the instruction in an amount of time which is variable andwhich is dependent upon a type of the instruction. In one embodiment,the execution time for certain types of instructions is predetermined,but varies with the different types of instructions. The execution timefor other types of instructions may be based on completion of ahandshake procedure. Timing the execution of instructions may involvegenerating pulses of a variable-frequency clock signal. The delaybetween pulses may be determined by passing pulses through selectabledelay circuits that have different delays.

Numerous other embodiments are also possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention may become apparent uponreading the following detailed description and upon reference to theaccompanying drawings.

FIG. 1 is a diagram illustrating simple request/acknowledge handshakeprotocol scheme, as well as the transmission and reception of thehandshake signals by input/output ports of a processor in accordancewith one embodiment.

FIG. 2 is a block diagram of a processor according to one embodiment ofthe invention.

FIG. 3 is a block diagram showing more detailed view of the registersand the ALU of the processor in one embodiment.

FIG. 4 is a block diagram showing a detailed view of the clock generatormodule of the processor in one embodiment.

While the invention is subject to various modifications and alternativeforms, specific embodiments thereof are shown by way of example in thedrawings and the accompanying detailed description. It should beunderstood, however, that the drawings and detailed description are notintended to limit the invention to the particular embodiment that isdescribed. This disclosure is instead intended to cover allmodifications, equivalents and alternatives falling within the scope ofthe present invention as defined by the appended claims.

Detailed Description of Preferred Embodiments

One or more embodiments of the invention are described below. It shouldbe noted that these and any other embodiments described below areexemplary and are intended to be illustrative of the invention ratherthan limiting.

As described herein, various embodiments of the invention comprisesystems and methods for executing program instructions in a dataprocessor at a variable rate. In one embodiment, a processor isconfigured to examine received instructions and to identify an executiontime associated with each instruction. For example, instructions may beassociated with one of three types that are require short, medium, orlong times to execute. The types may be named “fast,” “medium” and“slow,” respectively. The processor generates clock pulses that arerouted through fast, medium or slow delay circuits to achieve theappropriate execution time for the corresponding instruction. Theprocessor is also configured to identify instructions which areinput/output (I/O) instructions and have execution times that aredependent upon completion of handshake procedures instead ofpredetermined delays.

In a conventional synchronous design, the clock period consists of themaximum or worst-case timing through the longest logic path, plustop-level clock skew and uncertainty, all over the worst-case operatingconditions relative to process, voltage and temperature variations. Thisinvention allows the processor to run at “best-case” speeds for a giveninstruction at current operating conditions. There is no globalclock—instead, each instruction is examined to determine the appropriateclock period for that instruction based only on the timing paths thatproduce the needed results. All other timing paths are ignored.

Pipelining is a common method that is used to improve processorperformance. Registers are placed at various stages in the logic paths.The performance is improved compared to a non-pipelined design becausethe clock period can be shortened. Without pipelining, the clock periodmust be greater than the total path delay, plus uncertainty and margin.With pipelining, only the path delays between each set or stage ofregisters is considered. The clock period can then be reduced to that ofthe worst stage, plus uncertainty and margin.

The timing variation of different paths through a module for differentoperations can be dramatic. If the clock period were not a fixedquantity, and could vary based on the actual path of interest for theoperation of interest, performance could be improved, possibly as analternative to pipelining. For example, consider the followinghypothetical instruction examples:

-   -   a) move data from one register to another    -   b) increment the value in a register    -   c) multiply two registers and store the result

The path delay required for a) is relatively short—from the sourceregister through any routing multiplexers, to the destination register.The path delay for b) would be somewhat longer, though relatively short.The source register would be routed through an ALU that would performthe increment function, with the result being routed to the destinationregister (which in this case is the same as the source). The path delayfor the third example, c) is significantly longer. While similar to b),instead of an adder, the sources must go through the logic paths of aparallel multiplier. Not only are the logic paths of variousinstructions quite different, the incidence of their use within programsvaries widely as well. In is quite common to have more shorter, simpleroperations or instructions than the longer, more complex operations incommon blocks of code.

One of the principle concepts of processors is the reuse of hardwareover time. As an alternative to connecting together the hardwareelements needed to implement a function, software is written, renderingthe function into an algorithm. The sequence of events needed toimplement the algorithm is translated into instructions that are thenexecuted over time on a single piece of hardware, the processor. Time ismarked by a clock signal with a specific frequency or clock period. Theinstruction stream dictates the sequence of operations, including loopsand branches that must be performed. Hardware costs are reduced andflexibility is attained at the expense of performance. As semiconductorprocess technology continues to advance, hardware speeds also continueto increase. Device densities increase, as do the costs and complexitiesof the actual hardware design. Therefore, the increasing trend of movinghardware designs to software becomes more attractive for a larger set ofapplications.

In early processor systems, memory was expensive and slow compared tothat available today. One method used to improve performance was to tunethe instruction set to accomplish more with fewer instructions, thususing fewer, more complex instructions. This reduced the number ofaccesses to slower external memory, thereby improving performance. These“complex instruction set computers”, or CISC processors then usedinternal microcode subroutines to break those complex instructions intothe native microinstructions. The use of complex instructions layered ontop of microcode led to some instructions taking multiple clock cyclesto execute. At the system level, they were more efficient since theyused fewer instructions, which meant fewer accesses to slower, expensiveexternal memory.

Today, due to advances in semiconductor process technology, memory ismuch cheaper and faster, with large amounts of it available on the samedie as the processor. The internal logic of the processor is faster aswell. One strategy to improve performance has been to utilize on-chipmemory as a cache and simplify the instruction set. These processors areknown as “reduced instruction set computers” or RISC processors. Byeliminating microcode subroutines, the entire instruction flow may thenbe pipelined. This results in many cascaded stages of much smalleroperations, resulting in a higher overall clock frequency. Thethroughput (of straight-line instruction sequences) is thus greatlyaccelerated. Multiple parallel execution units are also sometimesemployed. Situations such as program branches or conditional jumps cancomplicate pipelined systems, however. Therefore, in addition to heavypipelining, contemporary high-end processors often employ speculativeexecution, register renaming, and other very complex performancetechniques to help offset the effects of non-sequential instructionexecution or dependencies in sequential instructions that are executedin parallel. These steps are necessary to keep the very long pipelinesfilled and running, as stalling the execution pipelines causes severeperformance penalties. In addition to the added complexity, thesetechniques also increase area and power consumption significantly. Theclock network of a device can comprise a significant portion of thepower budget. With heavy pipelining comes even larger and faster clocknetworks.

Practical architectures need not strictly adhere to the either the CISCor RISC approach, but may combine features of both. Some situationsrequire backward compatibility to earlier processors in the productfamily, thus requiring CISC instruction support. This may be handlednatively, or through emulation via the native instruction set of theunderlying hardware.

Processor performance can be defined as the number of instructionsconsumed over a period of time. Decreasing the clock period (byincreasing its frequency), and/or increasing parallelism (of execution)are common methods of improving performance.

Power consumption is becoming an increasingly important design metric.Just handling the distribution and dissipation of power can be asignificant challenge for high performance chip designs. The increasingrole of portable electronics that operate from a battery sometimesdrives the application requirements. Other applications may be concernedwith the maximum power density for an expansion slot in a chassis. Formany applications, asynchronous approaches provide the necessaryperformance while conserving power.

When considering the fundamental problem of implementing a function inhardware, and whether or not to use a software approach with aprocessor, area, power consumption, and other tradeoffs must beconsidered.

Asynchronous logic has the following advantages:

-   -   a) Low noise    -   b) Reduced power    -   c) Best possible versus worst-case speed    -   d) Scaling over operating conditions (voltage, temperature,        process)

Asynchronous logic has the following disadvantages:

-   -   a) Design flow    -   b) Design tools    -   c) Testing

Asynchronous logic does not have a global clock or timing reference.There are a number of methods of accomplishing this, ranging frominserting delays in signal lines, to providing completion signaloutputs. Terminology commonly used to classify various asynchronousdesign approaches include:

-   -   a) Self-timed—similar to synchronous, but with a locally        generated clock    -   b) Speed invariant—signals have handshakes between registers; a        delay is inserted into the handshake signals based on the delay        through the logic between registers.    -   c) Delay-insensitive—unbounded delay for gates and interconnect        requiring handshake

One embodiment incorporates aspects of all three. For instructions oroperations that do not involve input or output (such as communicationwith external logic or other processors), the processor is speedinvariant since a delay is selected based on the lumped path delay forthe type of operation or instruction selected. The speed invariant logicis used to generate a clock only for the desired destination registers,so it also contains aspects of a self-timed approach. When performinginput or output operations, the processor is delay-insensitive sinceeverything is on indefinite hold until the appropriate handshake isperformed.

This embodiment provides a processor that is simple, fast, and powerefficient, so that it may be used to replace blocks of conventionallogic. The blocks of logic may be simple, or complex, depending on thealgorithm and throughput requirements. Multiple instances of theprocessor may be connected together, or interspersed with conventionallogic in a design.

As described in this embodiment, asynchronous operation and simplicityare the keys to performance. Some asynchronous designs use storageelements in a pipelined arrangement. In this invention, all pipelinesare removed, and each instruction is completed in a single cycle. Theentire processor is in a known, stable state at the end of eachinstruction. There are no side effects or hazards as a result ofinstruction order or sequence, and no latencies are introduced.

Instructions are grouped into either internal or external types.Internal instructions operate on data held within the processor and keepresults there. External, or I/O instructions are ones where data isinput or output from the processor. In keeping with asynchronousmethodology, all external instructions must have a completion signal,defined by the chosen communication protocol. A simplerequest/acknowledge handshake protocol scheme, as shown in FIG. 1, hasbeen incorporated in the preferred embodiment. A generic request (REQ),acknowledge (ACK), and data (DATA) waveform is shown on the left side ofthe figure. Input ports are provided in the processor for receivinginput data (IDATA), an input request (IREQ) input signal, and forproviding an input acknowledge (IACK) output signal. The directions arereversed for output ports in the processor, which provide an outputrequest (OREQ) and output data (ODATA) signal outputs, and receive anoutput acknowledge (OACK) input signal.

An external device, another processor, or other logic wishing to provideinput data would supply data on the IDATA port and assert the IREQ inputsignal. When the processor encounters the appropriate input instruction,the IACK signal would be asserted (provided that the IREQ signal isactive). The system remains in this state until the IREQ signal isremoved, at which point the IACK signal is also de-asserted and the nextinstruction is executed. It may be possible to have various inputinstructions. For example, one version may wait for valid input data assignified by the IREQ signal, while others may only sample the IREQsignal and move on if there is not valid input data.

Similarly, when the processor wishes to output data to an externaldevice or other logic, it provides the data on the ODATA port whileasserting the OREQ signal. The OACK input signal is then continuouslysampled, and the processor stays in this state until the OACK signal isasserted. Once the OACK signal is asserted, the OREQ output signal isde-asserted, and the machine waits for the OACK signal to becomeinactive. Once that occurs, the machine continues on with the nextinstruction. Note that there may also be variations of outputinstructions that provide static values not requiring a handshake orsimilar protocol.

While the protocol illustrated in the preferred embodiment is simple andeffective, other variations of input and output signaling are possibleand acceptable as long as it is possible to determine completion.

FIG. 2 is a block diagram of a processor according to one embodiment ofthe invention. While the processor shown in the figure and describedbelow represents one embodiment, many variations are possible that stillincorporate the novel aspects of this invention.

For simplicity, the processor is shown with only a single input port 25and a single output port 21 that interface to external logic, devices,or additional processors. Any number of input or output ports may beeasily accommodated with this architecture. The registers 27 hold inputor output data, along with any intermediate results or other data. Anynumber of registers may be implemented in a given design. They may bediscrete registers, or incorporated as a register file. An optional datamemory 26 may be used to provide additional storage, and perhaps reducethe number of registers required. The output of the registers feed thearithmetic-logic unit (ALU) 22. Data in registers is fed into the ALUwhere it is processed, with the results going back to registers.

The results of many ALU operations have side effects or conditions thatmay be useful to subsequent instructions. Examples include the sign bit,the carry output from addition, or other similar notifications. Theseare stored in the FLAGS 23 module. A program counter PROG CNTR 24represents the execution sequence of the machine by providing addressesto a program memory PGM MEM 210. The program memory in turn dispensesthe instruction pointed to by the program counter. The FLAGS 23 may beoptionally used to provide non-sequential program flow, as qualifiersfor jumps, branches, or the like. An example would be “jump if notzero”, where the zero flag is examined. Not explicitly shown are otheroptional paths that could provide data to the program counter fornon-linear program execution, such as the use of signed or unsigneddisplacements interrupt traps, or other similar mechanisms. The programmemory 210 may be fixed or programmable. A fixed program memory could beimplemented as a ROM, PLA, or other similar structure. A programmablememory could be changed dynamically, or just at initialization time.

The instruction decoder INSTR DEC 28 decodes the instruction from theprogram memory 210 to provide control signals to other internal modules.The outputs from the instruction decoder are varied and include controlsfor data routing multiplexers, ALU operation codes, and the like. Theclock generator CLK GEN 29 also receives a field from the instructiondecoder to generate a clock pulse for the program counter and also toclock any specified destination registers. It should be noted that theclock generator in this embodiment does not generate a conventionalclock signal, but instead generates pulses that may be needed byconventional synchronous registers that are incorporated into thedesign.

FIG. 3 shows a more detailed view of the registers 27 and the ALU 22. Inthe design example shown, there are four registers, labeled R3 throughR0. The registers contain a full word, which is a function of theparticular processor implementation. Each register has a dedicated clockpulse signal, RCLK3 through RCLK0, which are derived from a commonsource in the clock generator module 29. At the end of the currentinstruction, only the clock signals for designated destination registerswill be pulsed, producing a clock edge. Depending on the instruction,the source and destination registers could be identified implicitly, orby bit fields within the instruction. For example, aregister-to-register move instruction could contain a 2-bit source anddestination register field, where the two bits would be binary encodedto represent one of R0 through R3. The bit field used to determine thedestination register would also be used to enable the appropriateregister clock pulse. Alternatively, an instruction could use predefinedregisters. In FIG. 3, the registers are shown as having common inputdata—more complex arrangements are also possible. For example, fullprecision multiplication on two inputs of the same word size produces aresult that is twice the word size. Therefore, a multiplicationinstruction could multiply R0 and R1, placing the result in R2 and R3implicitly, with no need to explicitly name the source or destinationregisters. In this case, the result would straddle two registers, withR3 being the most significant, and R2 being the least significant. Inother words,R3:R2=R0×R1

In this case, at the end of the multiply instruction, both RCLK2 andRCLK3 would produce edges.

The registers R3 through R0 have input data IDATA as well as the ALUoutput as possible sources. The ISEL signal, from instruction decoder28, is used to select ALU data versus IDATA via multiplexer (mux) 32.IDATA would be selected for instructions that accept input data.Instructions that produced output data ODATA would select the sourceregister by setting OSEL appropriately, which would then provide thecorrect output data via output multiplexer 31. The ALU has many variedpaths from the two data inputs to the data output. The A input of theALU is determined from multiplexer 32, which is controlled by the ALU_Asignal. Similarly, the ALU B input is chosen by multiplexer 33, which iscontroller by the ALU_B input signal. The ALU_A and ALU_B signals areoutputs from the instruction decoder 28. Perhaps the shortest path wouldbe for simply moving data from one register to another, in which theinput data of one port on the ALU would be reflected on the ALU output.Other paths involving operations of one or more inputs resulting in anoutput exist, with varying path lengths based on the complexity of theoperations.

A key aspect of this embodiment involves timing the logic paths from theregister outputs, through the ALU, to the register inputs for each ofthe possible instructions. Depending on the variance and clustering ofthe path delays, a number of bins are created for grouping instructionswith similar delay values. The number of bins is arbitrary, as is thenumber of paths or instructions associated with each bin. Much of thisis a function of the instruction set and the design of the ALU. Theinstruction decoder then identifies the bin with which an instruction isassociated when the instruction is decoded. For the sake of discussion,three bins will be defined for the present embodiment: slow, medium, andfast. The variation between the bins is not necessarily, and in fact isunlikely to be, linear. For example, the register-to-register moveoperation previously mentioned would likely be categorized as fast,while the multiply would probably be slow. Input or output operationsare not classified according to one of the bins. The completion timingof an input operation is based on the availability of data, coupled withcompleting the communication protocol. Similarly, the completion of anoutput operation is based on the receiving device or logic to acceptingthe output data, along with properly completing the protocol sequence.

FIG. 4 contains a detailed view of the clock generator module 29. RSTand ENABLE are global control signals that were not shown on previousdiagrams. RST is an asynchronous reset signal, and is provided toinitialize the processor. ENABLE will allow the processor to beginoperation when active, provided RST is not asserted. When the processoris starting up, the Enable Logic 42 supplies a priming edge to PulseGenerator 41, which generates a single pulse that is fed into a seriesof delay circuits labeled SLOW, MED, and FAST. The delay circuitsconsist of similar structures as the logic gates and paths themselves,and so their delay tracks the logic path delays over process,temperature, and operating conditions. One of the delay circuits ischosen based on the current instruction by signal DLY_SEL and theidentification of the bin with which the instruction is associated.

The clock output CLK is active at the end of every instruction, and isused by the program counter PROG CNTR 24. All other registers have theirown independent clock signals that are only enabled if the registersneed to be updated. The clocks for registers R3 through R0 are shown asRCLK3 through RCLK0. The CLK output is fed back to Pulse Generator 41 tocreate additional pulses. The module group in the lower left of thefigure is actually part of the instruction decoder INSTR DEC 28 module.The instruction is input to the instruction decoder, where it is decodedand drives much of the control logic in the processor. IN_INSTR is asignal that, when asserted, signifies that the current instruction is aninput instruction. Similarly, OUT_INSTR signifies that the currentinstruction is an output instruction. The speed of the currentinstruction, (slow, medium, or fast) is signified by the delay selectDLY_SEL signal, which is a binary encoded selector. IO_INSTR is reallyjust the logical OR of IN_INSTR and OUT_INSTR, which is active when thecurrent instruction is either an input or an output instruction. Thecommunication protocol signals input acknowledge (IACK) and outputacknowledge (OACK) are used if the instruction is an input or outputinstruction. The instruction decoder also supplies decodes for registersthat need to be updated or are destinations of the current instruction.DST_R3, DST_R2, DST_R1, and DST_R0 each signify that R3, R2, R1 and R0,respectively, need updating. As can be seen in the figure, each islogically AND-ed with the clock by gates A3 through A0 to generateindividual register clock signals for the registers.

Enable logic 42 is used to prime the clock generation sequence. Thepulse generator 41 takes an input signal with a rising edge and shapesit so that the high period of the waveform has the proper width neededfor proper operation. Normally, the output clock CLK is used to drivethe pulse generator in a feedback arrangement. During the initializationsequence, before the first instruction has been executed, the enablelogic provides this function.

The multiplexer 43 is used to select the event that terminates thecurrent instruction cycle: either the handshake for an I/O operation, ora delayed clock pulse for an internal operation. For internaloperations, the DLY_SEL signals are used to control multiplexer 44 toselect one of the timing delays, which are slow, medium, and fast inthis case. The actual delays are based on the worst-case timing path forinstructions in that bin or timing range, and are generated by passingpulses generated by pulse generator 41 through circuitry that delays thepulses by different amounts.

Those of skill in the art will understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and the like that may be referenced throughoutthe above description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof. The information and signals maybe communicated between components of the disclosed systems using anysuitable transport media, including wires, metallic traces, vias,optical fibers, and the like.

Those of skill will further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented invarious ways. To clearly illustrate this variability of the system'stopology, the illustrative components, blocks, modules, circuits, andsteps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented in theparticular functional blocks specifically described above depends uponthe particular application and design constraints imposed on the overallsystem and corresponding design choices. Those of skill in the art mayimplement the described functionality in varying ways for eachparticular application, but such implementation decisions should not beinterpreted as causing a departure from the scope of the presentinvention.

The benefits and advantages which may be provided by the presentinvention have been described above with regard to specific embodiments.These benefits and advantages, and any elements or limitations that maycause them to occur or to become more pronounced are not to be construedas critical, required, or essential features of any or all of theclaims. As used herein, the terms “comprises,” “comprising,” or anyother variations thereof, are intended to be interpreted asnon-exclusively including the elements or limitations which follow thoseterms. Accordingly, a system, method, or other embodiment that comprisesa set of elements is not limited to only those elements, and may includeother elements not expressly listed or inherent to the claimedembodiment.

While the present invention has been described with reference toparticular embodiments, it should be understood that the embodiments areillustrative and that the scope of the invention is not limited to theseembodiments. Many variations, modifications, additions and improvementsto the embodiments described above are possible. It is contemplated thatthese variations, modifications, additions and improvements fall withinthe scope of the invention as detailed within the following claims.

1. A device comprising: a processor; wherein the processor is configuredto execute each of a plurality of instructions in an amount of timewhich is variable and which is dependent upon the instruction types. 2.The device of claim 1, wherein the instruction types include at least afirst instruction type and a second instruction type, whereininstructions of the first instruction type are executed by the processorin a first amount of time and instructions of the second instructiontype are executed by the processor in a second amount of time.
 3. Thedevice of claim 2, wherein the instruction types further include aninput/output instruction type, wherein the processor is configured toexecute each instruction of the input/output instruction type in anamount of time that is based on completion of a handshake procedure. 4.The device of claim 1, wherein for each instruction received by theprocessor, the processor is configured to identify the correspondinginstruction type and to select the amount of time for execution of theinstruction that corresponds to the identified instruction type.
 5. Thedevice of claim 1, wherein the processor is configured to terminateexecution of an instruction based on a clock pulse that is delayed by aselectable amount of time.
 6. The device of claim 1, wherein theexecution of each instruction is timed by pulses of a variable-frequencyclock signal.
 7. The device of claim 6, further comprising a pluralityof delay circuits, wherein each delay circuit is configured to delaypulses of the variable-frequency clock signal by a different amount oftime.
 8. The device of claim 1, wherein the processor is configured tooperate alternately in either a first mode in which each instruction isexecuted in an amount of time that is selected based on the type of theinstruction, or in a second mode in which each instruction is executedin an amount of time that is dependent upon execution of a handshakeprocedure.
 9. The device of claim 8, wherein the processor operates inthe first mode when executing internal instructions and operates in thesecond mode when executing input/output instructions.
 10. A methodimplemented in a data processor comprising: receiving a programinstruction; and executing the instruction in an amount of time which isvariable and which is dependent upon a type of the instruction.
 11. Themethod of claim 10, further comprising if the instruction types is afirst type, executing the instruction in a first amount of time, and ifthe instruction types is a second type, executing the instruction in asecond amount of time.
 12. The method of claim 11, further comprising ifthe instruction types is an input type, beginning execution of theinstruction by performing an input handshake procedure, and if theinstruction types is an output type, completing execution of theinstruction by performing an output handshake procedure.
 13. The methodof claim 10, further comprising, for each instruction received by theprocessor, identifying the corresponding instruction type and selectingthe amount of time for execution of the instruction that corresponds tothe identified instruction type.
 14. The method of claim 10, furthercomprising timing execution of one or more instructions with pulses of avariable-frequency clock signal.
 15. The method of claim 14, furthercomprising generating the variable-frequency clock signal by delayingeach successive pulse by a selectable amount of time.
 16. The method ofclaim 15, wherein delaying each successive pulse by a selectable amountof time comprises selectively routing successive pulses of the clocksignal through two or more delay circuits, wherein each of the delaycircuits is configured to delay the pulses by different amounts of time.17. The method of claim 10, further comprising operating alternately ineither a first mode in which each instruction is executed in an amountof time that is selected based on the type of the instruction, or in asecond mode in which each instruction is executed in an amount of timethat is dependent upon execution of a handshake procedure.
 18. Themethod of claim 10, further comprising operating in the first mode whenexecuting internal instructions and operates in the second mode whenexecuting input/output instructions.